This invention relates to acoustic laminates and panels and more specifically, to laminates comprising epoxy resin impregnated glass fabrics.
A variety of acoustic sandwich constructions are known in the art. These generally consist of an impervious backing sheet and a porous face sheet separated by a foam or compartmented layer, such as a honeycomb core. Typical of these acoustic panels is that disclosed by B. T. Hulse et al., in U.S. Pat. No. 3,166,149 and T. S. Crispin et al, in U.S. Pat. No. 3,822,762. Often these panels include a porous cover sheet which, while sometimes decorative, adds undesirable weight to the assembly while reducing the acoustic efficiency of the system.
While some of the prior art acoustic laminates and panels have been relatively effective, problems remain both in the structures and methods of manufacture. Resin characteristics, such as quantity of resin, flow and wetting characteristics, etc., affect panel properties. In some cases resin may block pores, reducing acoustic properties, while in others which limit resin flow to a point where porosity is retained, structural characteristics of the acoustic laminate are poor, resulting in delamination or unbonded conditions when the panel is subjected to loading.
Polyimide resin impregnated glass fabric acoustic laminates of the sort described in U.S. Pat. No. 3,502,171, have been used, especially in high temperature applications. However, these materials have a number of disadvantages when their high temperature characteristics are not required. Because of the high temperature cures required and the release of volatiles during cure, these materials cannot conveniently be cured on plaster or other similar tooling. Both the raw material cost and the cost of fabrication (due to the required high temperature cure and postcure, and the necessarily elaborate bleeder system) for polyimide materials are undesirably high.
Attempts have been made to use polyester and epoxy resins, which do not release significant volatiles upon curing and have inherently greater flow than polyimides. However, problems have been encountered in controlling acoustic and/or structural properties as a consequence of the basic resin characteristics. Thus, these resins have not come into practical use.
Therefore, there is a continuing need for improvements in acoustic laminates and panels.